Furthermore, MECs initiate different signal cascades for response to different pathogen infections [18,19], for instance, activating the TLR2/TLR4-NF-κB pathway for response to E. coli infection, while activating the TLR2/IL17A pathway response to S. aureus and the TLR2/TLR4-IL-1β/IL-6 pathway response to mycoplasma. Here, TLR4 is linked to infection.